Recently, a demand for high recording density has been increased on a magnetic disk to be mounted on information processing devices such as a hard disk drive. Under such a situation, a glass substrate excellent in flatness and substrate strength has been widely used in place of conventional aluminum substrates.
The glass substrate for a magnetic disk is produced, for example, by cutting out a doughnut-shaped circular glass plate (a circular glass plate having a circular hole at the center thereof) from a glass plate, cutting an inner peripheral surface and an outer peripheral surface by using a diamond wheel, thereafter successively performing main surface lapping and edge surface mirror polishing, and polishing the main surface of the circular glass plate with a polishing pad.
In order to increase the storage capacity of the magnetic disk, it is necessary to widen the recording area. It is therefore desirable that the main surface of the circular glass plate is flat to the more outer peripheral side. However, so-called roll-off usually occurs, which is a phenomenon that the main surface is gradually inclined to an edge hereof to decrease the thickness of the glass plate as shown in FIG. 1 which is a cross-sectional view schematically showing the vicinity of the outer peripheral surface of the circular glass plate after polishing. The more the portion where this roll-off occurs is enlarged, the more the recording area decreases.
The shape of the edge of the circular glass plate is evaluated by the duboff value. The duboff value means the maximum distance (the value in a cross section of the circular glass plate) from a straight line by which any two points selected in a radial direction of the circular glass plate are connected to a surface of the circular glass plate, as shown in FIG. 1. A duboff value nearer to 0 shows that the shape of the edge is good.
A range in which the duboff value is measured can be arbitrarily set, as long as it is a range of an outer peripheral edge of the main surface of the circular glass plate, that is to say, a range in which head flying is inhibited when used as a HDD disk. For example, in the case of a glass substrate for a magnetic disk having a diameter of 65 mm, the duboff value (duboff 2) measured in a range of R 30 mm (R is the distance from the center) to R 32 mm (0.35 mm to 2.35 mm from an edge A to the center as shown in FIG. 1) and the duboff value (duboff 1) measured in a range of R 29.9 mm (R is the distance from the center) to R 31.5 mm (0.85 mm to 2.45 mm from an edge A to the center as shown in FIG. 1) are employed. Incidentally, the edge A is an edge of the polished main surface, and usually also one edge of a chamfered surface. For the duboff value, reference can be made to JP-A-2007-257810, JP-A-2007-87533, JP-A-2007-250166, JP-A-2005-203021, JP-A-2006-92722 and JP-A-2006-99949.
In order to increase the storage capacity as described above, it is essential to decrease the duboff value, and polishing pads have hitherto been improved. For example, it has been proposed that polishing is performed by using a polishing pad in which the Asker-A hardness is increased to 90 or more by allowing a polishing material to be contained in a specified amount (see JP-A-2007-250166).